1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for acquiring frame synchronization in an asynchronous mobile communication system.
2. Description of the Related Art
The invention will be described herein with reference to a UMTS/3GPP (Universal Mobile Telecommunication System/3rd Generation Partnership Project) system, a next generation asynchronous mobile communication system. The 3GPP specification is silent as to the implementation of frame synchronization of a broadcasting channel (BCH). Herein, a description will be made as to a process of acquiring 10 msec-frame synchronization through initial cell search by a UE (User Equipment) in a UMTS system.
FIG. 1 illustrates a scheme for transmitting a primary synchronization channel (PSCH), a secondary synchronization channel (SSCH), and a common pilot channel (CPICH). Referring to FIG. 1, a UTRAN (UMTS Terrestrial Radio Access Network) transmits a primary synchronization code (PSC) CP for 1/10 period (i.e., 256 chips) at each slot of the primary synchronization channel. A UE then receives the primary synchronization code CP and synchronizes UTRAN time slots based on the received primary synchronization code CP. This is referred to as Cell Search Step #1.
The secondary synchronization channel is mapped with code group information Cst,1-Cst,15 of the UTRAN before being transmitted. After being time slot synchronized in Cell Search Step #1, the UTRAN detects the code group information and 10 msec-frame synchronization through the secondary synchronization channel. Here, the code group information of the UTRAN, information used to determine a code group to which the UTRAN belongs, is designated according to comma-free codes obtained by selectively combining 15 synchronization codes among 16 synchronization codes SSC1-SSC16. This is referred to as Cell Search Step #2.
As described above, the UE acquires 10 msec-frame synchronization through the initial cell search. Thereafter, the UE must rapidly demodulate a broadcasting channel (BCH) in order to quickly start a service. However, prior to demodulating the BCH the UE has acquired only 10 msec-radio frame synchronization of the PCCPCH (Primary Common Control Physical Channel), but has not recognized synchronization for 20 msec-frame demodulation of the BCH. In this state, if the UE demodulates the 10 msec-radio frame of the PCCPCH using a transmission time interval 20 msec (TTI) of the BCH and provides the demodulated frame to an upper layer, it is not assured that the 20 msec duration is time-aligned (or synchronized) with a 20 msec boundary of the BCH. A failure to align a first frame boundary causes transmission of erroneously demodulated data to the upper layer. One problem that arises is that in the case of an unsynchronized frame boundary, if user data existing in a cyclic redundancy check (CRC) position becomes equal to the CRC result, the UE will mistake the user data for normal data and transmit it to the upper layer.